1. Field of the Invention
The present invention relates to a switching apparatus, which is capable of being used as a switching apparatus selectively supplying electric power from a battery to respective loads, for example in an automobile.
2. Description of the Prior Art
In a conventional motor vehicle, a number of switching circuits are mounted in order to selectively supply electric power from a Battery to respective electric parts (hereinafter, the electric part is called "load") in accordance with an operation of an operational switch such as an ignition key, light switch, and audio switch, etc.
FIG. 19 shows a schema of such conventional art, in which a battery 1 is connected with a junction block (J/B) 2, the junction block 2 being connected with operational switches SW1, SW2, . . . located on an operational panel 3. The junction block 2 is provided with switching circuits according to the number of the operational switches SW1, SW2, . . ., each of the switching circuits bringing a connection between a power supply line from the battery 1 and an electric line linked to respective loads into conduction (ON state) or out of conduction (OFF state).
With this configuration, the battery's power is selectively supplied to respective loads through the junction block 2 in accordance with an operation of the operational switches SW1, SW2, . . . For instance, if a head light switch is operated to ON state, the power line from the battery 1 and the electric line going to head lights 4A and 4B are set to ON state, whereby the head lights 4A and 4B are supplied with electric power and the lights 4A and 4B are turned on.
It should be noted that such a system includes a variety of sub-systems, in some of which the power is directly supplied to the load thereof like the head light 4A and 4B and in others of which the power outputted from junction block 2 is supplied through the switching circuits 6A and 6B like e.g., motors 5A and 5B driving a power window. These switching circuits 5A and 5B are switching-controlled by operational switches 7A and 7B.
Practically, the junction block 2 is configured as shown in FIG. 20. The junction block 2 is provided with a plurality of relays L1, L2, L3, . . . The relays L1, L2, L3, . . . include one relay which is directly controlled to ON or OFF by the operational switches SW1 and SW2 such as, e.g., the relays L1, L2 corresponding to the head lights 4A and 4B mentioned above and the other relay which is embodied by a relay L3 controlled to ON or OFF in accordance with conditions of an ignition switch 8.
The relays L1, L2 is supplied with the battery's power from the battery through a fusible link (FL) 9 and fuses F1, F2. Thus, the fusible link 9 is fused when a large current having an amount equal to or greater than a permissible value flows into the power line connecting between the battery 1 and the junction block 2, and in turn, the fuses F1 and F2 are fused when an overcurrent having an amount equal to or greater than the permissible value flows into the electric line (wire harness) connecting between the junction block 2 and the respective loads, whereby making it possible to prevent the whole of the power line from smoke-producing and from igniting and to prevent an overcurrent from flowing into the loads.
Similarly, the relay L3 is supplied with the battery's power from the battery 1 through the fusible link 9, an output terminal of the relay L3 being connected with respective loads 5A, 5B through fuses F3, F4 and the relays L4, L5.
In the meantime, a high-performance and inexpensive semiconductor switch has been made readily available to someone as the development of semiconductor manufacturing technology has surged forward in recent years. With relation to such situations, a switching circuit using a semiconductor switch is proposed instead of the above mentioned relays L1, L2, . . . which operate by means of mechanical contacts.
This type of switching circuit generally has a protective function of protecting a semiconductor switch from overcurrent and overheat. More specifically, a semiconductor switch is protected from them by way of forcibly off-controlling a semiconductor switching when a current having an amount equal to or more than the current rating has flowed into the semiconductor switch or when the temperature of the semiconductor switch has soared into a temperature equal to or more than a prescribed value.
FIG. 21 shows an example of switching circuits using the semiconductor switch, which is embodied by a switching circuit 30 called "Intelligent Power Switch". The switching circuit 30 is connected in positions corresponding to the respective relays L1, L2, . . . instead of the respective relays L1, L2, . . . mentioned above. It will be mentioned as to, for example if the switching circuit 30 is connected in place of the relay L1. In this example, a power input terminal 12 is connected with a fuse F1 (See FIG. 20) while an output terminal 13 is connected with a load 4A. Further, a control signal input terminal 14 is connected with a operational switch SW1. It should be here understood that, in the case of the switching circuit 30, a control voltage generating section (not shown in figure) is practically provided between the operational switch SW1 and the control signal input terminal 14, which supplies the control signal input terminal 14 with a control voltage of e.g. 5 V as an ON control signal from the operational switch SW1 when the operational switch SW1 is operated to ON but does not supply the control signal input terminal 14 with the control voltage when the operational switch SW1 is operated to OFF.
The switching circuit 30 comprises a abnormal status signal generating section for informing the outside thereof that an abnormal condition has occurred in the switching circuit 30 on the basis of a value of an output voltage V.sub.OUT from the semiconductor switch. The abnormal status signal generating section 41 is connected with an abnormal status display unit 43 through a CPU (Central processing Unit) 42 as shown in FIG. 19, which detects that a semiconductor switch (.pi. MOS) 32 has been forcibly controlled to OFF by the action of the protective function of the switching circuit 30 in cases where the semiconductor switch 32 of the switching circuit 30 is supplied with an overvoltage, supplied with an overcurrent, or overheated, and then in response to the detection, sends an abnormal status signal to the CPU 42. The CPU 42 determines what has been gone wrong and which of the switching circuits 30 has been in an abnormal state on the basis of the abnormal status signal, and makes the abnormal status display unit 43 indicate the result of determinations.
A configuration of the switching circuit 30 having an arrangement of the intelligent power switch will be hereinafter described. The switching circuit 30 gives a .pi. MOS-FET 32 a power voltage VB through an input terminal 12 connected with the fusible link F1 (See FIG. 20), and performs an ON/OFF control of the .pi. MOS-FET 32 by a driver 33.
In addition, the switching circuit 30 is provided with: an overvoltage detecting circuit 34 for detecting the case that the power voltage V.sub.B corresponds to a certain overvoltage; a current detecting circuit 35 for detecting a overcurrent by comparing a value of a voltage obtained based on a value of a current flowing between a drain and a source of the .pi. MOS-FET 32 with a value of a reference voltage Vref outputted from a reference voltage generating circuit 33A; and a temperature detecting circuit 36 for detecting a overheating of the .pi. MOS-FET 32 by comparing a temperature voltage value V.sub.T obtained from a temperature sensor (not shown in figure) located near the .pi. MOS-FET 32 with the reference voltage Vref, wherein the detecting results of the respective detecting circuits 34, 35, 36 enter into a NOR circuit 37. Also, the NOR circuit 37 is supplied with a control voltage V.sub.IN through an inverter 38.
An output of the NOR circuit 37 is supplied to the driver 33 and a charge pump circuit 39. The charge pump 39 operates only if the output of the NOR circuit 37 is positive in logic. Specifically, if it is so, the charge pump 39 generates a driving voltage necessary to operate the .pi. MOS-FET 32 to ON by way of boosting a voltage of the power voltage VDD stabilized by a regulator 40, and supplies the driver 33 with the driving voltage. The driver 33 operates the .pi. MOS-FET 32 to ON by way of applying a driving voltage produced by the charge pump 39 to a gate of the .pi. MOS-FET 32 if an output of the NOR circuit 37 is positive in logic, while the driver 33 operates the .pi. MOS-FET 32 to OFF in the way of applying no driving voltage as above to the gate of the .pi. MOS-FET 32 if an output of the NOR circuit 37 is negative in logic.
In the switching circuit 30, in turn, an output voltage V.sub.OUT is supplied to the abnormal status signal generating section 41 through an inverter 44. The abnormal status signal generating section 41 comprises an n-channel MOS-FET 41A which operates to OFF if the .pi. MOS-FET 32 is controlled to ON and the output voltage V.sub.OUT presents high voltage. As opposed to such operations, the MOS-FET 41A operates to ON if the .pi. MOS-FET 32 is controlled to OFF and the output voltage V.sub.OUT presents low voltage. An drain terminal 41B of the MOS-FET 41A is pulled up.
The CPU 42 (See FIG. 19), therefore, can determine that the protective function is inoperative in the switching circuit 30 (that is, no abnormal status occurs) when no potential difference exists between the drain terminal 41B of the MOS-FET 41A and the source terminal 41C of the same. On the contrary, when a certain potential difference exists between the drain terminal 41B and the source terminal 41C, the CPU 42 can determine that the protective function is operating in the switching circuit 30 (that is, it is in abnormal status).
Although the above-mentioned switching circuit 30 provides an overheat-detection circuit 36 to protect the semiconductor switch (.pi. MOS FET32) from thermal destruction, the heat from that semiconductor switch has not only such thermal destruction but also other adverse effects, leading to a variety of failures in such types of conventional switching circuits with poor protections.
For example, such a switching device has been proposed that has a switching circuit provided with the above-mentioned semiconductor switch's over-current protection and overheat protection features, as well as an anti-fuming function for the cables (wire harness) connecting the semiconductor switch and loads (see for example Japanese Patent Application No. H.-8-149623). This switching device will detect an electric current flowing through a semiconductor switch to monitor the current value itself and also other time-wise factors with a microcomputer and, if a large current flows to cause fuming from the harness, turned off the semiconductor switch, thus preventing the harness from fuming without providing any fuse in the downstream of the semiconductor switch.
However, this type of switching device does not consider the harness's fuming characteristics which change with the ambient temperature and the heat from the semiconductor switch, so that even when the harness is still largely yet to begin fuming that device may turn off the semiconductor switch or, oppositely, even when a large current flows so as to cause fuming, that device may not turn it off.
By the above-mentioned switching circuit 30, if the semiconductor switch 32 gets a rapid rise in temperature above a threshold temperature of the overheat detection circuit 36, the overheat protection mechanism functions to protect the switch 32 from overheating; however, when the switch 32 is kept at a temperature a little below the threshold one for a long time (for example, when it is kept at 140 degrees Celsius against a 150-degree threshold), the performance of the switch 32 may gradually deteriorate.
When the semiconductor switch 32 is kept at a relatively high temperature, a heat therefrom would transfer to other circuitry, thus causing an increase in temperature of the circuits such as a driver 33, over-current detection circuits 34 and 35 etc. shown in FIG. 21 actually having on/off control over the semiconductor switch 32 (hereinafter called a protection circuit because this circuitry functions to protect the semiconductor switch from over-current and overheating), so that this protection circuit may deteriorate in performance or malfunction. Such deterioration in the performance of the protection circuit would directly lead to the malfunctioning of the load supplied with power from that switching circuit, thus deteriorating the reliability of the entire system including the switching circuit.
If a plurality of switching circuits 30 are contained in a junction box, a rise in temperature of the semiconductor switch 32 of any one of those circuits 30 may have adverse effects on the other switching circuits 32. Moreover, the junction box, usually placed in the vicinity of the engine room of a car, may easily get heat from the engine, which is combined with the heat from the switching circuit 30 to deteriorate the switching circuit 30's performance.